j6  Mr.  hutton  on  the  initial 
-out,  was  equal  to  the  length,  of  the  chord  of  the  arch  de- 
fended by  the  bottom  of  the  pendulum. 
This  defeription  may  convey  a general  idea  of  the 
nature  and  principle  of  the  experiment ; but  belides  the 
center  of  ofcillation  and  the  weights  of  the  ball  and  pen- 
dulum, the  effect  of  the  blow  depends  alfo  on.  the  place 
of  the  center  of  gravity  and  the  point  of  impact : it  will, 
therefore,  be  now  neceffary  to  give  a more  particular  de- 
feription of  the  machine,  and  of  the  methods  of  finding 
the  abovementioned  requifites,  and  then  inveftigate  our 
•general  rule  for  determining  the  velocity  of  the  ball,  in 
nil  cafes,  from  them  and  the  chord  of  the  arch  of  vi- 
bration. 
■Of  the  particular  defeription  of  the  machine , and  of  the 
determination  of  the  centers  of  gravity  and  ofcillation . 
Tab.  i . Fig.  i . is  a reprefentation  of  the  machine  ufed 
in  the  firft  three  courfes  of  experiments;  and  fig.  2.  of  that 
which  was  ufed  in  the  other  two.  I fhall  here  deferibe 
the  former  of  thefe,  and  afterwards  take  notice  of  the 
few  particulars  in  which  the  other  differs  from  it  when  I 
come  to  treat  of  the  ufe  of  the  latter. 
The  firft  pendulum  confifted  of  a block  of  found  and 
dry  elm,  being  nearly  a cube  of  twenty  inches  long, 
which 
